And... We're through!
Had some time this morning and thought this would be a productive way to spend it.
Little collage of the works today in getting through the wall attached.
I've had to come into the house about 1ft sideways from where it looks like the fibre is going to pop out of the ground due to where the steps change level, which would have meant locating the hole below the recommended 300mm height above ground level. I guess they'll just run it over the surface.
I drilled to 10mm diameter initially with a slight incline, but my new Superrod magnet chain attachment won't go through anything like a 10mm hole, so I threw caution to the wind, and stuck the world's longest 22mm SDS drill bit in my 10.8V Bosch handheld hammer drill (I only had access to a borrowed 10mmx400mm and 22mm x lord knows how long without having to buy a new drillbit). Surprisingly the little Bosch didn't seem to much care and was through in a few seconds.
Put one Superrod with the flexible end and the magnet attachment through from the inside into the space under the stairs, and put the chain on the end of another couple of superrods pushed though the cavity wall and into the space under the stairs from the outside. It took literally two goes and about 2 minutes to pick up the chain with the magnet end. Once I'd pulled the rod through, attached a 3mm Nylon paracord doubled up and pulled it back through the hole (so effectively I have 2 pull cords in the hole. The SuperRod magnet / chain kit is awesome, well worth the £11 cost
https://www.amazon.co.uk/Super-Rod-CRCM-Chain-Magnet/dp/B0018D7F6U - though you do need to have a set of SuperRods to use it).
Next up just got to get some 22mm OD flexible pipe and run it over the pull cord, which should be able to guide it well enough I think to be able to grab it inside.
I think we're all set.